The present invention relates, in general, to methods and apparatus for generating intense ion beams, and more particularly to methods and apparatus for generating ion beams in the megavolt range and for controlling and concentrating the beams so generated.
The production of high current electron beams has been accomplished in the past, typically by the application of a high voltage pulse in the range of 0.5 to 10 MeV of 50 nanoseconds (nsec) length, to a diode consisting of closely spaced metal anode and cathode surfaces. The high voltage extracts from the cathode surface electrons which are accelerated across the diode gap in the form of a beam. The electron current across the diode is described approximately by the Child-Langmuir law for space-charge-limited flow if account is taken of the fact that the effective cathode-anode gap decreases rapidly during a pulse as a result of plasmas created adjacent the anode and cathode, which plasmas expand during the course of the pulse to reduce the spacing. The electron current produced in this manner finds extensive application in the art, particularly as a high energy source for producing X-rays.
Such electron beams are limited in some uses, because the mass of an electron is extremely small. It was recognized, however, that if a high current could be produced using more massive particles such as ions, the resulting beams would have a number of important applications, for an intense ion beam could be used to rapidly heat a plasma to fusion temperatures, and could find use in nuclear studies for producing intense neutron fluxes, isotope fluxes, and the like. For example, a fusion reaction can be produced in an isotope of hydrogen by the application of pressure to increase the density of the isotope to 100 - 1000 times its normal value, thereby creating heat which results in a nuclear fusion reaction. Literally millions of atmospheres of pressure are required to produce such a reaction, however, and a source of energy for accomplishing this has long been sought. High powered beams of particles such as electrons were considered, but it was anticipated that the required pressure could better be applied by high energy, intense beams of particles such as ions.
An attempt was made to generate such an ion beam by utilizing the diode arrangement from which electron beams were derived, and to this end voltages in the range of 0.5 to 10 MeV were applied across closely spaced diode plates in pulses of approximately 10.sup.-7 seconds duration. However, at such high voltages and such short durations, it was found that electrons would be pulled out of the cathode surface readily, but that ions could not easily be pulled out of the solid metal anodes. In operating such devices, it was found that an electron diode will under very high voltage conditions produce a plasma on the anode, and thus provide a source of ions which can be accelerated toward the cathode. This was, however, an inefficient way to produce an ion beam, since less than 2.5 percent of the available energy was put into the ions, the remainder being utilized to accelerate electrons from the cathode surface. This is because, for the same energy and current, the ions move more slowly and constitute a larger space charge, and the space charge for both ions and electrons limit their flow.
In order to obtain the desired ion flow, it was found that it would be necessary to provide a suitable material at the anode that would readily release ions. At the same time, means would be required to suppress the relatively light and easily moved electrons, for the normally occurring electron current utilized most of the pulse energy. Thus, the problem was faced as to how to produce a suitable source of ions within the time limit of the applied pulse; that is, in less than 10.sup.-8 seconds, and also how to produce a significant flow of such ions in such a way as to produce a recoverable, and thus useable ion beam current. Consideration was given to the use of lasers or other energy sources to create a layer of plasma at the anode of the diode device, which plasma could then serve as a source of ions. These and other considerations failed to produce the desired results, however, because the flow of electrons in such devices consumed virtually all of the available power and effectively prevented the flow of ion current.
An early solution to some of the foregoing problems was the provision of an arrangement of the type diagrammatically illustrated in FIG. 1. In this type of device, the anode is constructed of a mesh which is highly transparent to electrons, preferably passing approximately 95 percent of the electrons accelerated towards it. The mesh is made of a damage-resistant material such as tungsten, and is coated with a material that will provide the required source of ions. For example, an absorbed monolayer of H.sub.2 could be utilized to provide a source of Hydrogen ions, or protons, while other coating materials such as hydrocarbons may be utilized for this purpose. Two cathode plates at ground potential are located symmetrically, one each side of the anode, and thus the device may be referred to as a triode, although it is more in the nature of a double diode. A pulsed voltage applied to the anode causes electron field emission so that a space charge limited electron current leaves each cathode. About 5 percent of this current collides with the anode and constitutes an electron current drain for the device. The remaining electrons continue through the anode to the opposite cathode. The current leaving each cathode is about 1/2 the Child-Langmuir current since the returning electrons effectively double the space charge between the cathode and anode, so that the total current approaching the anode is the current that would result from a vacuum diode of spacing d and voltage V.
The electrons which collide with the anode produce a plasma around the anode which is transparent to the high energy electrons. Ions are emitted from the plasma at the space charge current density appropriate to the effective spacing of the diode electrodes and are accelerated towad both cathodes. This continues until the plasma has expanded to fill the gap between the electrodes and shorted out; about 50 - 100 nanoseconds. With this construction, the electron current is reduced by a factor of 20, and for a source of hydrogen ions (protons) about one-third of the available energy is put into the ions. Further, since there is an excess of electrons around the cathodes, and the electrons need only a small energy to follow the ions, the flow of ions constitutes an electrically neutral plasma, making it possible to propagate the resulting flow of ions through a vacuum with the divergence properties of the beam being determined by the initial direction of emittance of the ions. The principle of operation of a triode such as that illustrated in FIG. 1 is further described in an article entitled "Generation of Intense Pulse Ion Beams" published in Applied Physics Letters, Vol. 25, No. 1, July 1, 1974, pages 20 - 22.
In a further development of the triode of FIG. 1, it was found that only one real cathode is necessary for the operation of the device, provided that the triode is surrounded by a region of ground potential. Upon removal of one of the cathodes, electrons emitted by the remaining cathode and passing through the anode to enter the vacant side will, by their own space charge, produce a virtual cathode at their turning point, which will be at substantially the same distance from the anode as the real cathode. This simplifies construction of the triode, and allows extraction of ions from the vacant side through a perfectly transparent cathode. This construction is referred to herein as a reflex triode.
Although the reflex triode discussed above produced improved results, it was found that a substantial loss in energy occurred adjacent the edges of the cathode, for electrons emitted at these areas travelled quickly to the support structure for the anode, which structure is opaque to electron flow, and thus these electrons constituted a current flow which consumed a significant part of the power applied to the device. To prevent this loss, a magnetic field parallel to the triode axis, with lines of flux extending across the anode-cathode gap, and of a sufficient strength to prevent electron drift outwardly from the mesh portion of the anode is provided.
As indicated above, the loss of electrons by impact on the anode mesh can be utilized as a source of energy for formation of a plasma. The metal mesh material may be coated with hydrocarbons which have been found to be good proton sources. However, coated metal anodes are generally slow in the production of plasma, compared to the pulse length of the applied voltage, and a better solution has been found to be the use of initially nonconducting meshes of hydrocarbons such as nylon. It has been found that at the beginning of a pulse, large electrical fields are produced along the nylon threads which make up the mesh. These fields are believed to cause a surface breakdown which rapidly produces a plasma that is then maintained by the circulating electrons. The operation of a reflex triode is described in an article entitled "Advances in the Efficient Generation of Intense Pulse Proton Beams" published in the Journal of Applied Physics, Vol. 46, No. 1, January, 1975, pages 187 - 192.
Although reflex triodes of the type described above have been useful in confirming the theory that an ion flux can in fact be produced in a controllable and predictable manner, nevertheless such devices left unsolved numerous problems relating to the practical applicability of such ion streams. For example, the reflex triode operation produces a flow of ions over a relatively large area at a relative low power level. This left open the question of how to collect the ions so produced and to focus them into a beam which could produce useable amounts of energy at a predetermined target.